The present invention relates in general to ultrasonic systems, and in particular to methods and circuitry for driving an ultrasonic transducer.
Ultrasound technology is utilized in a variety of applications from machining and cleaning of jewelry crystals to performing surgical operations involving for example clearing obstructed blood vessels, to disrupting or lysing cells in order to release the inracellular contents (e.g., nucleic acid). The basic concept of ultrasonic systems involves the conversion of high frequency electric energy into ultrasonic frequency mechanical vibrations using transducer elements. Such systems typically include a driver circuit that generates electrical signals which excite a piezoelectric transducer assembly. A transmission element such as a probe connects to the transducer assembly and is used to deliver mechanical energy to the target.
For a given user-defined parameter (e.g., amplitude level) there is a resonance frequency at which the driver circuit operates most efficiently. The driver circuit is thus designed to operate at resonance frequency for a particular application. In many applications, however, due to changes in the environmental conditions the optimal resonance frequency drifts as the mechanical energy is being delivered. Such varying environmental conditions may include, for example, changes in temperature or the consistency of the target itself. The challenge, therefore, is to design an ultrasonic system that adapts to such environmental variations such that the driver circuit operates at its optimal resonance frequency at all times.
The present invention provides methods and apparatus for implementing an ultrasonic system that dynamically detects and maintains peak operational resonance frequency. In one embodiment, the invention dynamically sweeps the output frequency range to locate the peak load current. The resonance frequency corresponding to the peak load current is used as a reference frequency in a control loop such as a phase-locked loop (PLL). The control loop includes a voltage-controlled oscillator (VCO) that is controlled by a loop controller such as a microprocessor and operates to lock onto the dynamically sensed reference frequency. In response to the VCO output, a pulse-width modulator (PWM) circuit drives a pair of switches that adjust transducer current to maintain the circuit locked on the resonance frequency at a substantially constant current. By combining the frequency sweeping feature that locates the peak load current and the resonance frequency, with the microprocessor controlled pulse width modulated current switches, the invention provides for an ultrasonic system that maintains a substantially constant displacement of the transmission element with maximum efficiency. The invention further provides an algorithm that allows the user to specify parameters such as amplitude level of the driver, and then performs a multi-step frequency sweep to drive the transducer in one of several modes including constant current drive, constant voltage drive and constant power drive. In various specific embodiments, the invention provides additional features such as optional circuit alarm and VCO linearity compensation.
Accordingly, in one embodiment, the present invention provides an ultrasonic system including: a transducer coupled to a secondary of a transformer; and a control loop coupled between the transducer and a primary of the transformer, wherein the control loop includes a current sense circuit coupled to the transformer and configured to detect load current; a loop controller coupled to the current sense circuit and configured to dynamically set a loop reference frequency in response to the sensed load current; a voltage-controlled oscillator (VCO) coupled to the controller and configured to generate an output signal oscillating at the reference frequency; and a pulse-width modulator coupled to the VCO and configured to control an amount of current in the primary of the transformer.
In another embodiment, the present invention provides a driver circuit for an ultrasonic transducer, wherein the driver circuit includes: a current sense circuit coupled to detect a transducer load current; a controller coupled to the current sense circuit and configured to set a reference frequency corresponding to peak resonance frequency; a voltage-controlled oscillator (VCO) coupled to the controller and configured to generate an output signal oscillating at the reference frequency; and a pulse width modulator coupled to the VCO and configured to modulate an output current of the driver circuit. The pulse width modulator includes a first switch and a second switch whose operation is controlled by pulse width modulated signals generated in response to the VCO output signal.
In yet another embodiment, the present invention provides a method for driving an ultrasonic transducer, wherein the method includes (a) sweeping a frequency range of the output to locate a peak load current; (b) defining a reference frequency as the frequency corresponding to the peak current; (c) adjusting an oscillation frequency of an oscillator to the reference frequency; (d) controlling output transistor switches by pulse width modulated signals generated in response to the oscillator output to adjust transducer current; and (e) periodically repeating steps (a) through (d) to dynamically adjust the reference frequency that controls the transducer current.
The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the present invention.